1. Field of the Invention
The invention relates to materials and methods for fabricating thin film electrical components in integrated circuits and, more particularly, liquid precursor solutions which are used in misted deposition apparatus for depositing thin films.
2. Description of the Related Art
As is well-known in the art, the electrical components in integrated circuits are made up of layers of thin films which are connected by wiring layers and separated by insulating layers. Simple thin film materials and compounds, such as silicon glass, have been formed using a liquid deposition process. Complex compounds, i.e., compounds containing more than two elements, in the prior art have always been formed using processes such as vacuum sputtering (i.e., E-beam, D.C., R.F., ion-beam, etc.), laser ablation, reactive chemical vapor deposition including metalorganic chemical vapor deposition (MOCVD); and liquid application methods using sol-gels (alkoxides) or carboxylates. However, none of these known methods have been able to produce metal oxides with properties that are entirely satisfactory for use in integrated circuits. In all of the prior art processes, except sputtering, the films produced had significant physical defects, such as cracking, peeling, etc.
It was substantially impossible with the conventional processes, particularly sputtering, to reliably and repeatably produce metal oxides with a specific stoichiometry within tolerances required for integrated circuits. Some processes, like MOCVD, could be dangerous or toxic. Most required high temperatures that were destructive to an integrated circuit, and provided poor "step coverage" of a substrate to be covered, e.g., the prior art techniques resulted in a relatively excessive build-up of deposition of the film at the boundary of any discontinuities on the substrate. In prior art liquid deposition processes, it was impossible to control thickness with the degree of accuracy that is required to manufacture integrated circuits. As a result, up to now, metal oxides and other complex materials have not been used in integrated circuits except for one or two specialty, relatively expensive applications, such as the use of sputtered lead zirconium titanate (PZT) in ferroelectric integrated circuits that were expected to have short life times.
U.S. Pat. No. 5,456,945 to McMillan et al. provided a substantial advance in the art by teaching the use of ultrasonic transducers to generate a volume of mist or aerosol into a deposition chamber. Within the deposition chamber, a DC voltage is applied between a substrate holder and a barrier plate for induced polarization in the aerosol particles. A constant flow of carrier gas, e.g., argon, is used to transport the aerosol to an integrated circuit substrate or wafer for the deposition of liquid thin films. These liquid thin films are dried and annealed to yield the thin films of an integrated circuit.
When the precursor is processed in misted deposition apparatus according to the No. 5,456,945 patent, the selection or choice of a liquid precursor affects step coverage, liquid film deposition rate, and the final thin film morphology. The use of some liquid precursor solutions results in no material being deposited as desired. Other solutions provide poor step coverage, and yet other solutions have good step coverage with poor film morphology. There is a need for liquid precursor solutions that function in misted deposition apparatus to provide good step coverage and good film morphology. There is also a need for parameters permitting the design and development of precursor solutions that will function appropriately in misted deposition apparatus.